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2. The Atom
2.1 Evidence for the existence
of the electron
• The universe consists of energy & matter.
Matter occupies space and has mass.
• Law of conservation of mass states
matter is neither created nor destroyed in
the course of a chemical reaction
• All matter composed of particles (atoms,
molecules or ions)
• Evidence is shown by expt involving the
diffusion of ammonia gas and HCl gas in
a glass tube
Diffusion
Diffusion: is the movement of
gas from areas of high gas
concentration to areas of low
gas concentration
Definition!
Diffusion of ammonia and HCl
gases
2.1 Evidence for the existence
of the electron
• Atoms very minute particles
• e.g. H atom in its normal state has
radius of about 4 x 10-11m
• The atom is made of even smaller
particles
• The atom is considered to be the
basic unit from which all substances
are formed
2.2 History of the
atom
Greeks
• Some ancient Greeks
noticed that
substances such as
iron, gold, lead, silver
were 'pure'
• They could not be
broken down into
simpler substances
by heat.
400 BC
• The Greek philosophers
were the first to propose
that matter is made up of
tiny particles.
• They didn’t do any
experiments to back this
up.
Greeks
The Greek word for ‘indivisible’ is
‘atomos’ from where we get the
word atom to describe these
indivisible particles
A Brief History of Chemistry
• Over the next two millennia, major advances in
chemistry were achieved by alchemists. Their
major goal was to convert certain elements into
others by a process called transmutation.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
The Greeks
History of the Atom
• Not the history of atom, but
the idea of the atom
• The Greeks tried to
understand matter
(chemicals) and broke them
down into earth, wind, fire,
and air.
~
~
• Democritus and Leucippus
Greek philosophers
Greek Model
“To understand the very large,
we must understand the very small.”
Democritus
• Greek philosopher
• Idea of ‘democracy’
• Idea of ‘atomos’
– Atomos = ‘indivisible’
– ‘Atom’ is derived
• No experiments to support
idea
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Democritus
DEMOCRITUS (400 BC) – First Atomic Hypothesis
Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos.
Properties of atoms:
• indestructible.
• changeable, however, into different forms.
• an infinite number of kinds so there are an infinite number of elements.
• hard substances have rough, prickly atoms that stick together.
• liquids have round, smooth atoms that slide over one another.
• smell is caused by atoms interacting with the nose – rough atoms hurt.
• sleep is caused by atoms escaping the brain.
• death – too many escaped or didn’t return.
• the heart is the center of anger.
• the brain is the center of thought.
• the liver is the seat of desire.
“Nothing exists but atoms and space, all else is opinion”.
Some Early Ideas on Matter
Anaxagoras
(Greek, born 500 B.C.)
–Suggested every substance had its own kind of “seeds” that clustered together to
make the substance, much as our atoms cluster to make molecules.
Empedocles
(Greek, born in Sicily, 490 B.C.)
–Suggested there were only four basic seeds – earth, air, fire, and water. The
elementary substances (atoms to us) combined in various ways to make
everything.
Democritus
(Thracian, born 470 B.C.)
–Actually proposed the word atom (indivisible) because he believed that all
matter consisted of such tiny units with voids between, an idea quite similar to
our own beliefs. It was rejected by Aristotle and thus lost for 2000 years.
Aristotle
(Greek, born 384 B.C.)
–Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements
which combined as shown in the diagram (previous page).
Hot + dry made fire; hot + wet made air, and so on.
O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,
Who was Right?
•
•
•
•
•
•
•
•
Greek society was slave based
Beneath famous to work with hands
did not experiment
Greeks settled disagreements by
argument
Aristotle was more famous
He won!
His ideas carried through middle ages.
Alchemists change lead to gold
John Dalton
• 1808
• Proposed
an Atomic
Theory
John Dalton (Atomic theory)
.. the existence of atoms
• Developed a theory – that matter
is composed of tiny, indivisible
particles called atoms that can
neither be created nor
destroyed i.e. no change in mass
‘law of conservation of mass’
Conservation of Atoms
2 H2 + O 2
2 H 2O
John Dalton
H
H
H2
O
H
O2
+
H2
H
O
H 2O
O
H 2O
H
H
O
H
H
4 atoms hydrogen
2 atoms oxygen
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
4 atoms hydrogen
2 atoms oxygen
Legos are Similar to Atoms
H
H2
H
H
O
+
H2
H
H
O2
H
O
H 2O
H
O
O
H
H 2O
Lego's can be taken apart and built into many different things.
Atoms can be rearranged into different substances.
Conservation of Mass
High
voltage
electrodes
Before reaction
After reaction
glass
chamber
O2
High
voltage
H2O
O2
5.0 g H2
H2
80
0 g H2
g O2
300 g (mass
of chamber)
+
385 g total
45
? g H2O
40 g O2
300 g (mass
of chamber)
+
385 g total
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204
John Dalton proposed his atomic
theory.
1. All matter is made up of small
particles called atoms.
2. Atoms are indivisible – they cannot
be broken down into smaller
particles. False!
3. Atoms cannot be created or
destroyed. False!
FACT
 We now know that there are
particles within atoms –
subatomic particles e.g. protons,
neutrons and electrons.
 Radioactive atoms can change
into other atoms.
Dalton’s Symbols
John Dalton
1808
Daltons’ Models of Atoms
Carbon dioxide, CO2
Water, H2O
Methane, CH4
Structure of Atoms
• Scientist began to wonder what an atom
was like.
• Was it solid throughout with no internal
structure or was it made up of smaller,
subatomic particles?
• It was not until the late 1800’s that
evidence became available that atoms
were composed of smaller parts.
Discovery of the Electron
• In the 1800’s electricity was new,
exciting and the subject of a lot of
study.
William Crookes
• 1875
• Crookes passed an
electric current through
a complete vacuum just
to see what would
happen.
Crookes experiment
• Electricity seemed to leap across the
vacuum from the negative plate to the
positive plate.
• The radiation given off from the negative
cathode was called Cathode rays.
Crookes Tube
William Crookes
Crookes tube
(Cathode ray tube)
Glow
Cathode
(-)
Anode
(+)
Mask holder
William Crookes
A discharge tube
• A long glass tube fitted with a metal electrode
at each end
• When the gas inside is at a very low
pressure, passing electricity through the tube
results in invisible rays travelling in straight
lines from the cathode (-ve) to the anode
(+ve)
• Small objects placed in the path of the rays
cast a sharp shadow in the fluorescence at
the end of the tube
Crooke’s Vacuum tube
Maltese Cross
Cathode (-)
High
Voltage
Anode (+)
Vacuum tube means most of the air has been pumped out of it
The Effect of an Obstruction on
Cathode Rays
High
voltage
source of
high voltage
shadow
cathode
Anode
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
yellow-green
fluorescence
Crooke’s Vacuum tube
Cathode rays
Maltese cross
Shadow of cross
on glass
Cathode (-)
Anode (+)
An electric current passing through air causes the air to glow
Rays coming from the –ve electrode (cathode) cast a shadow
at the far end of the tube
• The cathode rays caused
shadows and could spin light
paddle wheels.
Crooke’s experiment
• This proved that the cathode rays
carried energy, and that they
might be made of particles.
• This also indicates that the rays
(particles) moved from the
cathode to the anode.
William Crookes
(cathode rays) (Hons)
• Showed some form of radiation passes
from the -ve electrode (cathode) to the
+ve electrode (anode) when a current
passes through a vacuum tube
-ve cathode
+ve anode
CNAP
• He called these cathode rays and was
convinced they consisted of particles
J J Thomson
• Was very interested
in the Crooke’s
discoveries.
• 1897 He used a
cathode ray tube that
had a positive and
negative plate
between which the
rays had to travel.
Background Information
•
•
•
•
Cathode Rays
Form when high voltage is applied across
electrodes in a partially evacuated tube.
Originate at the cathode (negative electrode)
and move to the anode (positive electrode)
Carry energy and can do work
Travel in straight lines .
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Cathode Ray Experiment
1897 Experimentation
• Using a cathode ray tube, Thomson was
able to deflect cathode rays with an
electrical field.
• The rays bent towards the positive pole,
indicating that they are negatively
charged.
CATHODE RAYS IN ELECTRIC
FIELDS
PROVIDES
ELECTRICAL FIELD
Thompson’s cathode ray tube
The Effect of an Electric Field on
Cathode Rays
source of
high voltage
High
voltage
cathode
negative
plate
_
+
anode
positive
plate
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
Thomson’s Experiment
-
voltage
source
+
vacuum tube
metal disks
Thomson’s Experiment
ON
-
OFF
voltage
source
+
Passing an electric current makes a beam appear
to move from the negative to the positive end
Thomson’s Experiment
ON
-
OFF
voltage
source
+
+
By adding an electric field…
he found that the moving pieces were negative.
Thomson
• He found the cathode rays were
attracted towards the positive
plate.
• The rays had to be negative (as
opposites attract).
Thomson 1897
Discovered the electron
• 1897
• Thomson announced that cathode
rays consisted of negative charged
particles.
• Thomson had discovered the
ELECTRON!
• (He called them corpuscles)
George Stoney
1891 UCG
• Proposed that the
smallest particle that
has a negative
charge should be
called an electron.
• So Thomson's
corpuscles were
called electrons.
An Irish scientist
Electrons
Cathode rays are streams of
negatively charged particles
called electrons
• But where had these tiny particles come
from?
• Since they were so small, Thomson
suggested that they could only have come
from inside atoms.
• So Dalton's idea of the indestructible atom
had to be revised.
1808 AD
• John Dalton proposed his atomic theory.
1. All matter is made up of small particles
called atoms.
2. Atoms are indivisible – they cannot be
broken down into smaller particles.
False!
3. Atoms cannot be created or destroyed.
• This discovery implied that Dalton was
wrong and the atom is not the smallest
particle of matter.
• It looked like the atom could be broken
down into smaller pieces – electrons.
JJ Thomson
• He did another experiment.
• He placed an electro magnet around the
vacuum tube.
• He changed the strength of the magnetic
field to match the force of the electric field.
• Using the magnetic field he was able to
bring the beam of electrons back down to
its original position.
Cathode Ray Experiment
JJ Thomson
Electromagnet
JJ Thomson (Hons)
• From the results of his expt:
• He was able to calculate the ratio of the
charge of an electron to its mass.
e
• This is the m of the electron.
JJ Thomsom (hons)
• But he couldn’t determine the
individual value of either number
• He found that these –vely charged
particles were about 2000 times
lighter than H atoms and were to be
found in all matter
JJ Thomsom
• Also proposed the ‘plum pudding’
model of the atom
• He visualised an atom as a sphere
of +ve charge into which –ve
electrons were embedded at
random
Thomson Model of the Atom
• William Thomson proposed that atoms consist of small,
negative electrons embedded in a massive, positive
sphere.
• The electrons were like currants in a plum pudding.
• This is called the ‘plum pudding’ model of the atom.
-
-
electrons
-
-
-
Plum-Pudding Model
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
Robert Millikan (Hons)
• Oil Drop Experiment
• Determine the mass
& charge of the
electron
Actual apparatus used
Robert Millikan
• 1911, Millikan used the ‘Oil Drop’ expt to
measure accurately the charge on the
electron (see book)
• This then allowed its mass to be
calculated
• The mass is much smaller than that of any
atom
• Confirming that the atom is made up of
smaller particles
Millikan’s oil drop expt
Millikan’s oil drop expt
• He sprayed tiny drops of oil between two
charged plates.
• He used x-rays to cause the molecules in
the air to lose electrons.
• As the oil droplets passed through, they
picked up electrons from the air. i.e. the
oil droplets were now –vely charged
Oil droplets -
Molecules in air
Electrons -
Millikan’s oil drop expt
Oil Drop Experiment
oil droplets
+
Small hole
-
Charged plate
. . ... ................................... .
...... . ........ ........... .....................................................................
..... ..... . . ........ ..... .................................... .. oil atomizer
........ ... . ............................................ .
. ..... . . . .. .......... .. .... .
.. . . . .. . . . .
.. ..........
X-rays
..
.
.
.
...
.............. .. . .
................... ... .
.. .......................... .
.... .............. ..
...... ..........................................
............................... ... .. . .
.. ....... ..... ..... .. .....
............ . .................. .
...... .... ..
............... .. .. . .
.. . .... .
Charged plate
Robert Millikan
Telescope
oil droplet
under observation
Millikan’s oil drop expt
• By attaching a battery to the
plates, he created an electric field
between the plates that would act
on the charged oil drops .
• i.e. the –ve oil droplets were
attracted to the +ve charged plate
Millikan’s oil drop expt
• He adjusted the voltage
• till the electric field force would
just balance
• the force of gravity on an oil drop,
• and the oil drop would hang
suspended in mid-air.
When the oil
droplet was
stationary, the
weight of the oil
droplet due to
gravity must have
been equal to the
electrical force
pulling the droplet
upwards
Robert Millikan
•
Using this information , He was able to
calculate:
1. The size of the charge on
the electron
2. The mass of the electron.
JJ Thomson
• Since atom is neutral
• There must be a
positive charge to
neutralise the negative
electrons.
JJ Thomson
• He developed the
idea that
• atoms are made of
negative electrons
embedded in a gel of
positive charge
• (a "plum pudding"
model).
Other pieces
• Proton - positively charged pieces
– 1840 times heavier than the electron
• Neutron - no charge but the same mass as
a proton.
• How were these pieces discovered?
• Where are the pieces?
1909
• Scientists began to question
the simple view of atom.
Ernest Rutherford
(The modern view of the atom was developed by Ernest Rutherford)
Discovered the nucleus & proton
• Worked in JJ
Thomson’s lab
• overturned Thomson's
atom model in 1911
• with his gold foil
experiment
• in which he
demonstrated that the
atom has a nucleus.
Ernest Rutherford
• Two of Ernest Rutherford's
students, Hans Geiger and Ernest
Marsden, were doing an
experiment at Manchester
University with radiation.
Ernest Rutherford
• They were using the dense,
positively charged particles
(called alpha particles)
• as 'bullets' to fire at a very thin
piece of gold foil.
Rutherford ‘Scattering’
Lead collimator
Gold foil
a particle
source
q
GOLD ATOM
• They expected the
particles to pass straight
through the gold atoms.
Experiment method
• Bombarded a thin leaf of gold foil
with alpha particles (+vely
charged)
• Used a zinc sulfide screen to
detect scattered alpha particles
Rutherford’s Goil Foil Experiment
Experiment Results
Gold atom
Ruherford’s experiment
• Most alpha particles were
observed to pass straight
through the gold foil.
• They had passed through
basically empty space!
Rutherford’s experiment
• A few, were scattered at small
and large angles (90°), and
some even bounced back
toward the source.
Interpreting the
Observed Deflections
.
.
.
.
.
.
beam of
alpha
particles
.
.
.
.
.
undeflected
particles
.
.
.
.
.
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
.
deflected particle
Density and the Atom
• Since most of the particles went through,
the atom was mostly empty.
• Because the alpha rays were deflected so
much, the positive pieces it was striking
were heavy.
• Small volume and big mass = big density
• This small dense positive area is the
nucleus
California WEB
Discovery of Nucleus
• Only a positively charged and
• relatively heavy particle,
• such as the proposed
nucleus, could account for
such strong repulsion.
New model of Atom
• Tiny, dense, positively charged
core called a nucleus,
• around which the light, negative
electrons, circulate at some
distance, much like planets
revolving around the Sun.
• Top: Expected results of
Rutherford's gold foil
experiment: alpha
particles passing through
the plum pudding model
of the atom undisturbed.
• Bottom: Observed
results: Some of the
particles were deflected,
and some by very large
angles. Rutherford
concluded that the
positive charge of the
atom must be
concentrated into a very
small location: the atomic
nucleus.
Rutherford’s
Gold-Leaf
Experiment
Conclusions:
Atom is mostly empty space
Nucleus has (+) charge
Electrons float around nucleus
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
Discovery of Proton
• Rutherford
continued to
bombard various
elements with
Alpha particles.
Ernest Rutherford (Hons)
• With light atoms e.g. Oxygen and
Nitrogen
• The alpha particles broke up the
nucleus to release positive
charged particles.
Rutherford
had discovered the
proton.
James Chadwick
Discovered the Neutron
• Bombarded a
sample of Beryllium
with alpha
particles.
Discovery of Neutron (Hons)
James Chadwick
• Some type of radiation with no
charge came from the
Beryllium.
James Chadwick
• The particles had the same
mass as protons but no
charge.
• He called them neutrons
Models of the Atom
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg
e
e
+
e
+
e
+
+
e
+e
+
e
e
+ e + e
Dalton’s
Greek model
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
-
- +
Rutherford’s model
(1909)
2.3 Properties of sub-atomic
particles
Particle
Relative
charge
Relative
mass
Location
Proton
+1
1
Nucleus
Neutron
0
1
Nucleus
Electron
-1
1/1838
‘Shells’
Recheck
•
•
•
•
•
•
•
•
•
Evidence for the existence of small particles
History of the atom
Discovery of the electron
Thomsom’s plum pudding model of the atom
Millikan’s oil drop expt
Discovery of the nucleus
Discovery of the proton
Discovery of te neutron
Properties of sub-atomic particles